Add tests and GitHub Actions

Replace the Travis tests, which used a separate repo and no longer
ran, with tests in this repo. Each case in test/cases is indented as
a whole and should be unchanged, with optional syntax, fold and tag
checks. The cases are from the previous tests, plus script-setup.vue
for <script setup> keywords and a sync check in oneline.vim.

test/run.sh runs the cases with Vim in a pseudo terminal, as syntax in
Ex mode isn't the same, and Neovim headless.

test/cases/template.vue

@@ -0,0 +1,23 @@
+<template>
+  <template>
+    <div></div>
+  </template>
+
+  <div></div>
+  <template>
+    <div></div>
+  </template>
+  <div></div>
+  <docs></docs>
+  <div></div>
+
+</template>
+
+<template>
+  <template></template>
+  <div></div>
+</template>
+
+<docs>
+# Hello
+</docs>